Interference patterns for making optical waveguide gratings, particularly fiber Bragg gratings, can be produced using an interferometer or a phase mask. Interferometers divide a coherent beam of light into two separate beams that are angularly recombined at the waveguides for producing a desired interference pattern. Phase masks, which are themselves diffraction gratings, divide a similarly coherent beam into different diffraction orders that are recombined at the waveguides for producing a similar interference pattern.
For purposes of manufacturing, phase masks are often preferred because interferometers can be less stable and difficult to use in production environments. Phase masks are more stable but have less flexibility for adjusting the period of their resulting interference patterns. The interference period between two collimated beams is a function of the wavelength of the interfering beams and the angle at which the beams are combined. Any change in the wavelength of a beam divided by a phase mask also changes the diffraction angles through which the divided beams are recombined, so a different phase mask is often needed for each desired interference pattern.
A few techniques have been developed to adjust the effect of photoinduced waveguide patterns produced by phase masks. Bragg wavelengths (center wavelengths) of the resulting grating responses are a function of both the grating period and the average refractive index of the waveguides. Small adjustments to the Bragg wavelength have been made by pre-straining waveguides (i.e., optical fibers) and by illuminating phase masks with converging or diverging beams.
The former technique is described in a paper entitled "Tuning Bragg Wavelength by Writing Gratings on Prestrained Fibers" by Quin Zhang et al., published in Photonics Technology Letters, Vol., 6, No. 7, July 1994. A photosensitive fiber is exposed to an interference pattern produced by a phase mask while under strain. When the strain is relieved, the Bragg wavelength is down-shifted with respect to a similarly exposed unstrained fiber. Only a limited amount of strain can be tolerated by fibers and other waveguides, so the amount of adjustment by this technique is limited.
A paper entitled "Magnification of Mask Fabricated Fibre Bragg Gratings" by J. D. Prohaska et al., published in Electronics Letters, Vol. 29, No. 18, Sep. 2, 1993, proposes to illuminate phase masks with converging or diverging beams to adjust the magnification of interference patterns incident to photosensitive fibers. The power of a converging or diverging lens, the distance between the lens and the phase mask, and the distance between the phase mask and fiber can be changed to adjust the magnification of the interference pattern within the Fresnel near field of the light passing through the phase mask. However, only small changes in periodicity are practical because interference patterns produced at a distance from the phase mask are limited by spatial coherence of the illuminating beam.
U.S. Pat. No. 5,327,515 to Anderson et al. mounts a lens between a phase mask and a photosensitive fiber to project an image of a interference pattern formed at the phase mask onto the fiber. The lens projection system can be arranged to provide magnification or reduction of the interference pattern projected onto the fiber. However, like the known interferometer arrangements, issues of stability and alignment render this technique less practical in a production environment.